1. Field of the Invention
The present invention relates to the non-invasive, on-line measurement of fiber flow and other fiber/mat properties in various fiber handling processes such as melt spinning, melt blowing, spunbonding, air laydown and wet laydown. More particularly, but not by limitation, the present invention relates to the use of laser Doppler velocimetry to measure the mass flux (defined as mass per area per time) of fibers, or filament velocity, in the fiber stream of a melt blowing apparatus.
2. Brief Description of the Prior Art
Since the construction of the first laser in 1960, the use of lasers in science, industry, and commerce has steadily risen. The use of lasers in the fiber industry is no exception. For example, a single laser beam, when shined upon a fiber, will produce a forward scatter pattern that is dependent on the fiber diameter. As a method of off-line, post-production fiber size measurement, use of this forward scatter pattern is more accurate and much faster than obtaining diameters by optical microscopy or scanning electron microscopy as shown by I. M. Fouda et al. in the article, "A Comparative Study of Three Methods for Measuring Fiber Diameter", Indian J. of Text. Tech., Vol. 13, pp. 11-16, 1988.
This forward scatter technique can also be used to measure the diameter of moving filaments. Thus, for example, the procedure can be used to monitor the diameter of a fiber during conventional melt spinning as indicated by A. A. Hamza et al., in "Production of Polyethylene Fibers and Their Optical Properties and Radial Differences in Orientation", Text. Res. J., Vol. 50, pp. 592-600, 1980. In addition, the back-scattered radiation from a moving (or stationary) filament can be used to measure refractive index as illustrated by H. M. Presby in "Refractive Index and Diameter Measurements of Unclad Optical Fibers", J. Opt. Soc. Amer., Vol. 64, pp. 280-284, 1974, and by J. M. Wilkes in "Calculating Fiber Index of Refraction from Laser Back-Scattering Data", Text. Res. J., Vol. 52, pp. 481-482, 1982. Besides the analysis of single filaments, single beam lasers can also be used to examine defects in yarns and fabrics. The randomness/texture of the surfaces causes the formation of a diffraction (speckle) pattern from which information on defects can be determined as shown by E. Toba in the article, "Determination of the Autocorrelation Function of Woven Fabrics Using Laser Speckles", Text. Res. J., Vol. 50, pp. 238-244, 1980. W. E. Wolf in the article, "On-Line Inspection for Nonwoven Fabrics", 12th Technical Symposium, Ass'n of Nonwoven Fabrics Industry, pp. 249-264, 1984, described a method for using lasers during post-production examination of nonwoven fabrics.
Another laser configuration that has found use in fiber science is laser Doppler velocimetry, or LDV. LDV is commonly used to measure fluid velocities by examining the Doppler shift in the laser light scattered by small particles moving with the fluid. In the usual dual-beam LDV system, two laser beams are focused so as to intersect and form a small measuring volume, or mv. Particles which pass through this mv produce Doppler bursts of scattered light. An LDV system is noninvasive and can measure velocities in the range of 10 .mu.m/s to 1 km/s.
In the textile area, LDV has been used for the measurement of the velocity of textile surfaces by W. Ringens et al., as indicated in the article "Optoelectronic Sensor for Motionless Velocity Measurement of Textile Surfaces", Textil Praxis International. Vol. 43, pp. 640-643, 1988. LDV has also been used for the measurement of fiber velocities in a fiber conduit during open end friction spinning by K. J. Brockmanns et al., in "Application of Laser Doppler Anenometry: Open-end Friction Spinning of Yarn", Textil Praxis International, Vol. 42, pp. 248-250, 1987, and by W. Bauer et al., in "Fiber Flow and Its Effects on the Characteristics of Open-end Spun Yarn", Textil Praxis International, Vol. 44, pp. 15-17, 1989. Lasers have also been used to measure the diameter of filaments and fiber bundles in-process by S. Holly and S. L. Chao in the article, "Noncontact Filament Diameter Measuring Technique", Instrument Soc. of America, 757 (pp. 1-7) (1974). Holly and Chao did not measure filament velocity to determine filament diameter but rather determined filament diameter by adjusting the interference fringe spacing to a minimum.
In the common application of LDV to a single-phase system (e.g., air or water), the fluid must often be artificially seeded with fine particles (e.g., oil droplets or a fine powder) in order for the laser light to be scattered and measured. The seed concentration is low and the seed sizes are small: 0.5 to 5 micron diameters are typical. A more complex problem is the application of LDV to two-phase flows such as air bubbles in water or large liquid drops in air as studied by F. Durst and M. Zare in "Laser Doppler Measurements in Two-Phase Flows", pp. 403-429, Proceedings of the LDA-Symposium, Copenhagen; Skovlunde, Denmark, 1975. In these two-phase flows, the size and concentration of the dispersed phase are much greater than the seed concentration in a single-phase system (of course, a seeded system is actually a two-phase system). Surfaces can be so large, in fact, that the mv intersection of the laser beams cannot illuminate the entire surface at one time.
For the particular case of spherical particles of one phase dispersed in a second phase, exact mathematical treatments are possible for relating the Doppler signals to the particle sizes. These treatments are discussed by W. M. Farmer in "Measurement of Particle Size, Number Density, and Velocity Using a Laser Interferometer", Applied Optics, Vol. 11, pp. 2603-2612, 1972; and by F. Durst and B. Ruck in, "Effective Particle Size Range in Laser Doppler Anemometry", Experiments in Fluids, Vol. 5, pp. 305-314, 1987. For cylindrically-shaped objects, such as fibers, the expected signals can be much more complex. If the cylinder moves with its major axis perpendicular to the plane of the crossing beams, the expected signals are very similar to the signals from a sphere. However, for a cylinder (i.e., fiber) with random orientation in space, prediction of the expected signals is mathematically intractable. Because of this, LDV has not previously been successfully applied to fiber technologies exhibiting random fiber orientations.